In a multi-antenna beamforming system such as a smart antenna system, a multi-input multi-output (MIMO, Multi-Input Multi-Output) system, and an active antenna system, the amplitude and phase are adjusted on different channels by using multiple transmit channels, and transmitted signals are superposed on an air interface to form different spatial beams. Due to many active and passive circuits on each channel and due to an unavoidable phase difference caused by factors such as local oscillation, transmit channels have different delays, amplitudes, and phases. Therefore, the channels need to be calibrated and the difference needs to be compensated, and signals of the same phase among air-interface combined signals can be superposed and signals of different phases among the air-interface combined signals can be offset according to expected directions, to from a desired transmission pattern and ensure normal working of transmitting beamforming in a multi-antenna system.
The channel calibration scheme in the prior art is shown in FIG. 1 and described as follows:
Generally, a probe signal whose power is far less than the power of a service signal is transmitted, for example, a probe signal whose power is lower than the power of the service signal by −30 dB, which is generally a pseudo random signal (PN, Pseudo Random Noise). A many-to-one combiner is used to combine signals on multiple channels by multiple feedback paths, and then a slide-window correlation matching with the transmitted probe signal is performed in a digital domain to detect the delay, amplitude and phase of each transmit channel.
In the process of implementing the present invention, the inventor finds at least the following disadvantages in the existing method of calibrating a channel by using a probe signal:
The probe signal causes interference to a service signal, where the detection and control of the interference is complicated. The power of a service signal fluctuates, and the power of a probe signal also needs to be adjusted adaptively to avoid impact on the service signal. The current high spectrum efficiency modulation modes, such as 16-Quadrature Amplitude Modulation (QAM, Quadrature Amplitude Modulation) and 64-QAM, impose a higher requirement on noise than the Quadrature Phase Shift Keying (QPSK, Quadrature Phase Shift Keying), and require lower impact caused by the probe signal. Such an adaptation process increases complexity of detection and control.